Silver-carbon brush block and method of making same



M. M. FAWZY Jul 'ls, 1969 SILVER-CARBON BRUSH BLOCK AND METHOD OF MAKING SAME Filed July 51, 1967 FIG! FIGZ

N m U m M? a M m a UNA-F. mm. M w T W -li TR T 1 0 mu; 5 R

TOP FACE TOP FA CE FA CE FACE DISTANCE Fm! FACE DISTANCE FROM TOP FACE OF Bl OCK FIGS 3,455,023 Patented July 15, 1969 3,455,023 SILVER-CARBON BRUSH BLOCK AND METHOD OF MAKING SAME Moharram M. Fawzy, East Hampton, N.Y., assignor to Rowe Industries, Inc., Sag Harbor, N.Y. Filed July 31, 1967, Ser. No. 657,394 Int. Cl. H01r 9/00 U.S. Cl. 29630 3 Claims ABSTRACT OF THE DISCLOSURE A process for making silver-carbon brush blocks wherein a compact pressed from a homogeneous silver-carbon mixture is heated under conditions such that a portion of the silver migrates to the outside of the block to provide a surface receptive to soldering and welding.

This invention relates to silver-carbon brush blocks and in particular to a method of making same.

Background of the invention Silver-carbon brush blocks are commonly used in various types of rotating components where a small contact voltage drop is desired between a sliding and a stationary contact. It will be appreciated that battery operated motors can tolerate but small external circuit losses and eflicient brushes are essential. Customarily, the brush block is mechanically and electrically attached to the stationary member. The composition of the brush block proper does not lend itself to mechanical means of attachment such as soldering, welding or peaning. This disadvantage dictates an intermediate means to be employed. One such common means is the comolding of a mass of essentially pure metal with the silver-carbon mixture during the press-forming of the block. This metal section is subsequently used for attachment purposes. In addition to the complication of handling two separate materials during the press-forming operation, an inherent cleavage surface exists between the metal and composition portions of the block and in practice, cleavage may occur at the interface. Another method is providing a silver coating through electro-plating or similar means. This latter process can result in contamination of the block composition by the solutions used in the process. Likewise, catastrophic failure of the contact surfaces may result. Further, this latter process requires the step of grinding the plating off the contact surface of the plated block before using.

Accordingly, it is an object of this invention to provide an improved brush block.

Another object of this invention is to provide a brush block having improved connection making qualities.

Still a different object of the invention is to provide an improved method of making brush blocks.

A further object is to provide a simplified method of making brush blocks.

These and other objects, features and advantages of the invention will, in part, be pointed out with particularity and will, in part, become obvious from the following more detailed description of the invention taken in conjunction with the accompanying drawing which forms an integral part thereof.

Brief description of the drawing FIG. 1 is a pictorial view of a typical brush block of this invention;

FIG. 2 is a pictorial showing of a brush block being uniformly heated;

FIG. 3 is a graphical representation of the silver-graphite distribution of a brush block heated as shown in FIG. 2;

FIG. 4 is a pictorial showing of a brush block being non-uniformly heated; and

FIG. 5 is a graphical representation of the silvergraphite distribution of the brush block heated as in FIG. 4.

Description of the preferred embodiments Referring now to the drawing, there is shown in FIG. 1 a typical brush block 10 with a conductive fiat leaf spring 11 soldered to the block 10. To make the block the silver and graphite powder components of the brush composition are prepared and thoroughly mixed in the desired proportions, suited for the final brush composition required. The composition is conventionally form-pressed into the shape required. The resultant blocks are hence subjected to a specific, uniform and closely controlled temperature lying within the range of 1000 F. to 1400 F,. for a specifically controlled length of time, lying within the range of 10 to 60 minutes for small brush blocks, e.g., /2 x W x 4 As shown schematically in FIG. 2, the block may be uniformly heated in a furnace. Under these conditions, part of the silver powder migrates from the central homogeneous composition to the outer regions of the block and forms layers that blend smoothly into each other.

FIG. 3 shows schematically the distribution of the silver relative to graphite. Progressing from the center of the block outwards, these outer layers become progressively denser in silver content and end in a thin layer of almost pure silver on the outside surfaces of the block.

The basic silver percentage in the composition as well as the thickness of the final silver layer desired dictates the choice of the temperature and time setting. The time and temperature settings individually influence the rate of film formation as well as the depth of film penetration. Within the ranges cited before, an optimum setting for each of these parameters can be experimentally found for each specific brush composition.

The optimum setting can readily be determined experimentally by persons familiar with brush blocks. Generally, the goal is to provide a brush block having a bonding surface which is mechanically secure, has low contact resistance and is not subject to excessive wear. Standard testing procedures can be used to determine these characteristics. By way of example, a composition containing 90% silver will require a shorter heating cycle to build up an outer layer of a given thickness than one containing 70% silver. Likewise, if a higher temperature is used for the 70% silver composition than for the 90% silver composition, then like results may be obtained in like times.

The silver film thus formed yields a surface which is suited to soldering or welding.

By subjecting one or more of the block surfaces selectively to heat, it is possible to selectively densify with silver one or more surfaces to a greater degree than the others.

Selective heating of one face may be obtained by assembling the blocks in a single layer sheet on a heat sink 12 and then heating the top surface of the assembled blocks. This may also be accomplished by resting the blocks on a ceramic plate which is heated. In either case the silver will migrate to the heated face. Heavy metal film layers can be so produced which can be used for mechanical securement to contact carrying members by peaning.

In one case parts by weight of powdered silver was mixed with 20 parts by weight of graphite. The mass was compacted by pressing with a pressure of about 20,000 psi. The compact was then heated for 35 minutes at 1075 F. 20 F. The resulting brush block exhibited excellent good soldering and welding characteristics and provided excellent wear.

While the .brush block does not normally require the inclusion of additives such as lubricants, such materials may be incorporated in conventional fashion where required. Certain silver alloys such as 60 parts silver: 40 parts copper may be substituted for the silver as required.

Various conductive forms of carbon which are generally used in making of brushes may be used in carrying out the invention. For low voltage application as for use in battery operated motors graphite is preferred.

Since the process does not employ any chemical solutions, no chemical contamination exists.

Since one homogeneous mixture is used, the form-pressing operation is simple and no inherent cleavage surfaces are generated.

Due to the thinness of the outer silver film combined with its characteristic gradual deep penetration, the following characteristics are realized:

(a) A greatly improved bond between silver film and composition proper, minimizing possibility of cleavage;

(b) The thin silver film at the contact surface permits deletion of silver removal operation in this area prior to usage. This procedure is mandatory with chemically deposited films due to their inherent thickness; and

(c) The thin film at the sides of the block improve the wear and life characteristics of the block as opposed to the heavier chemically deposited films which tend to groove and abrade the sliding contact.

There has been disclosed heretofore the best embodiment of the invention presently contemplated and it is to be understood that various changes and modifications may be made by those skilled in the art without departing from the spirit of the invention.

What I claim as new and desire to secure by Letters Patent is:

1. The method of making a silver-carbon brush block 4 having a solderable and weldable film of silver on the outside of the block comprising the steps of:

(a) mixing together silver and carbon particles until a substantial homogenous mixture results. (b) pressing said resulting mixture into a compact; and (c) heating said compact to a temperature between 1000 F. and 1400 F. for at least the period of time required to permit sutficient silver to migrate to the surface of the compact to form a solderable and weldable surface. 2. The method of claim 1 wherein said carbon is graphite.

3. The method of claim 1 wherein different faces of said compact are heated to unlike temperatures whereby said silver will migrate toward the hotter face.

References Cited UNITED STATES PATENTS 1,051,814 1/1913 Lowendahl 29630 2,507,780 5/1950 Gilbert 29-630 2,370,242 2/ 1945 Hensel et a1 29-630 2,729,559 1/ 1956 Matsukawa 75201 3,347,668 10/1967 Clark et a1. 75201 CHARLIE T. MOON, Primary Examiner R. B. LAZARUS, Assistant Examiner U.S. Cl. X.R. 

